Fac input in Blender
One of the big uses for this is for keyframing the Fac: value. In Blender, you cannot keyframe Node Tree Connections, so instead, you might have to fake it by keyframing a Fac: value.

Say that you have an animation, and you want your object to be there, be there, be there, and then suddenly be gone. Well, your first try might be having your shader connect to your Material Output and then keyframing a Transparent BSDF jumping to the Material Output connection at some point. That can't happen, though, so you can have your material and the Transparent BSDF fed into a Mix Shader, and keyframe (I while hovering over the Fac:) it to turn from 0.0 to 1.0 at a certain frame.